Recently, a technique has been proposed for electrolyzing water comprising a salt containing a slight amount of chlorine, e.g., sodium chloride or ammonium chloride, to obtain an oxidizing electrolytic solution having an extremely high oxidation-reduction potential. This electrolytic solution is known to exhibit very strong bactericidal and disinfectant activities, as well as cleaning activity and grain cohesion activity. Furthermore, the electrolytic solution which has been used for these purposes contains chloride ion only in almost the same concentration as ordinary tap water, and thus can be discharged as such without causing secondary pollution. Therefore, this electrolytic solution has recently found wide application, e.g., in hospital lavatories, in the cleaning of precision machines, and in the cleaning of semiconductors or liquid crystals.
A so-called strongly acidic water is required instead of mere acid water depending on the intended application of the acid water. Strongly acidic water is water or an aqueous solution having a pH value of not more than 3 and an ORP of not less than 1,000 mV. It is relatively easy to electrolytically produce an electrolytic solution having an ORP of not less than 1,000 mV. However, it is relatively difficult to electrolytically produce an electrolytic solution having a pH value of not more than 3. For example, the production of strongly acidic water by the electrolysis of an electrolytic solution containing chloride ion requires a reduction in pH by electrolysis by hundreds of times that required for producing hypochlorous acid by oxidizing chloride ion.
In other words, when a solution of a neutral salt such as a chloride is electrolyzed, the ORP of the electrolytic solution thus obtained is determined by the concentration of hypochlorous acid. The concentration of hypochlorous acid may be from about 1 to 5 ppm. If the concentration of hypochlorous acid exceeds the above defined range, chlorine gas is produced. Assuming that the current efficiency for producing chlorine gas is about 10%, the acidity of the electrolytic solution thus produced is not sufficient as represented by a pH of from 4 to 5. This is due to hydrochloric acid produced by the disproportionation reaction Cl.sub.2 +H.sub.2 O.fwdarw.HCl+HClO and hydrogen ion produced by the decomposition of water H.sub.2 O.fwdarw.H.sup.+ +OH.sup.- (cathode). In order to attain the desired pH value of 3 or lower, extra water electrolysis which neglects the current efficiency of chlorine production is required. Thus, extra electrolysis is required taking into account the chloride ion concentration.
This problem is no longer being studied and has been avoided by increasing the scale of electrolysis. However, raw water having an extremely low electric conductivity such as pure water and ultrapure water is required for semiconductor manufacturing. This requires a further increase in electric power. Furthermore, because prior art electrolytic systems had a small current density, a large electrode surface area was required. Because this electrode contacts the electrolytic solution, it is likely that the electrode component, which is a metal, further contaminates the system. This in turn contaminates the product solution by metallic components, which are the most undesirable type of contaminant.
In order to solve these problems, the present inventors proposed an electrolytic process which comprises applying a voltage across an anode and a cathode disposed in close contact with an ion-exchange membrane as a diaphragm such that the ion-exchange membrane acts as a solid electrolyte. In accordance with this electrolytic process, even if the current density is raised about ten times, e.g., to not less than 10 A/dm.sup.2, the required electrolytic voltage can be kept at a few volts. This makes it possible to effect electrolysis at a far lower voltage than in the prior art. Furthermore, the increase in current density makes it possible to reduce the required number of electrodes and hence reduce the size of the apparatus. In accordance with this process, electrolysis with the addition of a slight amount of an acid or salt makes it possible to obtain an acidic water having a high ORP, namely acid water from the anode chamber.
Although these conditions are useful for producing acid water for cleaning or bactericidal use, even further water electrolysis is required to lower the pH value of the acid water thus produced. Thus, the total required amount of electrolytic current remains much the same as in the prior art. Because the required electrolysis is excessive, contamination can easily occur. Even if this system can provide satisfactory properties at present, further improvements are needed when even more precise conditions are required.
Furthermore, this system is disadvantageous in that it takes too long to make adjustments during the dissolution of sodium chloride or the like in the anolyte. In other words, if sodium chloride is excessively added to the anolyte, the properties of the electrolytic solution can change and the excess sodium chloride or other salts can cause contamination. On the contrary, if sodium chloride or other salts are insufficiently added to the anolyte, the desired properties can hardly be obtained. In order to obtain the desired properties, prolonged electrolysis is required.
The present inventors also proposed a process of electrolysis in which an acidic chloride is supplied to the anode chamber. This process solves many prior art problems and reduces the size of the electrolytic cell. This proposal further makes it possible to reduce the required electric current to not more than 1/100 of the prior art systems. However, this process requires the addition of a chemical liquid such as hydrochloric acid having a controlled concentration to the anode chamber. This is disadvantageous in that it is necessary to uniformly diffuse a very dilute hydrochloric acid solution or the like throughout the anolyte, thus requiring a complicated control mechanism.